Hello,
I have been hosting my Wordpress site from my own local server. using www.ABC.com. but now i would like to host another site with new domain www.xyz.com on same server i have created site just fine using port 90 as 80 was in use by previous site. how can i forward www.xyz.com to this 90 port of my site.

Why would you think that? These are sites that are resolved on the public internet right. Who provides your dns for abc.com now? Your registrar, some dns service like zoneedit, dnsmadeeasy, etc. Just point your A record for www for xyz.com to your IP and there you go, where ever you host that dns at.